Think & Tinker, Ltd.
CNC router bits and carbide cutting tools with tutorials and technical guides

Shell Cutters and Inlay Pocket Cutters

Zero Glue Line

"Zero Glue Line". Talk about a Holy Grail. For years, craftsmen (and craftswomen) making top quality pool cues, and musical instruments have relied on years of experience and not a little luck to consistently inlay mother of pearl, abalone shell, turquoise, malachite, and precious metals into exotic hardwoods with no visible glue line. Given the vagaries of hardwoods, the difficulty of cutting brittle, abrasive minerals, and the absence of tools specifically designed to cut these materials, this amounted to nothing less than a heroic task.

Shell and mineral inlay media have most of the aggravating properties of very hard woods combined with an abrasiveness that quickly dulls just about anything you try to machine them with.

Glue Line inlay means that the inlaid component seems to be part of the wood in which it is embedded, as if the tree grew that way. The grain of the wood seems to originate and terminate in the inlay, with no intervening filler. This cue butt, designed and built by Keith Josey of Josey Custom Cues, is an excellent example of what can be accomplished when a master craftsman, with the right tools, sets out to create a functional work of art.

The problems encountered include edge chipping, rough sidewalls, and imprecise dimensions of the finished part.

For the past 9 years, Think & Tinker has been designing custom cutters for a wide variety of soft, non-metallic materials. In 1998, we began working with some of the best pool cue artists / craftsmen in the United States to develop a set of tools that would bring the precision of metal machining to CNC based woodworking. With the introduction of the 1/32" CueBit© SMF end-mill in 1999, this goal became a reality. Ongoing development has expanded the range of this family of tools from 0.005" (0.125mm) to 0.1250" (3.175mm).

Edge Chipping Cut walnut micrograph

The speed of the cutting edge and the chip load can be tuned with great precision in CNC based woodcutters. The sharpness and geometry of the cutting edge are another matter altogether. While you can always start with an new, sharp tool, the shape and attack angle of the cutter will determine how long the tool stays sharp and how well it cuts as it inevitably wears down. Historically, one way to tell when a tool was getting dull was to observe the degree of splintering and change the bit when the splintering became too severe. It is probably not possible to fabricate an end-mill that never splinters any kind of wood. However, proper design and testing coupled with fine tuning rotational speeds and feed rates make it possible to fabricate tools that will reduce splintering to such a degree that the splinters can be removed with a soft tooth brush as with this test cut in black walnut ( 0.0625" dia., 0.125" depth, 50 in./min. feed @ 40,000 RPM).

Rough Sidewalls Precision wooden gears

One aspect of metal cutting that has been missing from woodworking is the precision and accuracy of the shape and size of a finished part. At the heart of the problem lie the distorting effects of the grain of the wood and the inconstant density encountered as the cutter moves through the material. Wood, even hardwood, is a compliant media that tends to press away from a cutting edge and then swell back into place after the edge has passed. As a result, pockets tend to be smaller than expected and sidewalls end up in the wrong place. Efforts to overcome this tendency by increasing the attack angle and sharpness of the cutting edge, often result in a cutter that pulls into the wood, creating pockets that are bigger than desired. Making matters still worse, a poor match between the shape of the cutting edge and hardness of the wood can result in the grain being ripped apart instead cleanly cut. The SMF family of cutters are designed to cancel the tendency to push away from the wood by using a novel geometry that exerts a very slight pull into the wood, even when climb milling. The result is pockets that are exactly the size that they should be and edges so precise that clocks with wooden gears can be made to keep time with a precision of +/- 30 seconds a day (like the clock hanging in my office). All of the SMF tools are intended for full plunge operation eliminating the sidewall stepping commonly encountered with multi-pass cutting. Pretty neat, huh? ( 0.0313" dia., 0.125" depth of cut, 32 in./min. feed @ 40,000 RPM).